(1) Field of the Invention
The present invention relates to a Passive Optical Network (PON) in which a plurality of subscriber connection apparatuses share a part of an optical transmission line and, more particularly, to a WDM type PON system utilizing an Optical Wavelength Division Multiplexing (WDM) technique for multiplexing a plurality of optical signals with different wavelengths in the PON.
(2) Description of Related Art
A Passive Optical Network (PON) system is known as one of access network systems for point to multi-points (one to N) connection using optical signals. The PON system comprises a central office side apparatus OLT (Optical Line Terminal) placed at a facility center operated by a network vendor such as a type 1 carrier and a plurality of subscriber connection apparatuses ONUs (Optical Network Units) placed at user homes, respectively. An optical fiber connected to the OLT is diverged by a splitter (optical branching filter) into a plurality of branch optical fibers on the order of 8 to 64 lines. Each ONU is connected to one of branch optical fibers. The PON system enables significant reduction in the cost for optical fiber network construction because a plurality of subscribers can share the optical fiber section between the splitter and the OLT.
A wavelength division multiplexing type PON (WDM-PON) applies the wavelength multiplexing technique in the shared fiber section of the PON and following two schemes are known:
(a) A first scheme allocates different wavelengths to upstream and downstream signals and a plurality of ONUs use the upstream wavelength λu and the downstream wavelength λd in a time division multiplexing (TDM) manner. In this case, an optical power splitter operable without power supply is used as the splitter and each ONU transmits and receives data within a time slot allocated from the OLT.
(b) A second scheme allocates dedicated upstream signal wavelength and downstream signal wavelength to each ONU. In this case, the splitter needs a filter function to separate a downstream signal transmitted in a wavelength multiplexing manner from the OLT into individual wavelengths and to distribute them to respective ONUs. According to the second scheme, each ONU can get data addressed to it by receiving only the wavelength allocated to it beforehand.
The first scheme allows the use of optical transceivers that are common for all ONUs because there is only a difference between the upstream signal wavelength and the downstream signal wavelength. Further, this scheme is regarded as economical, since the optical power splitter is applicable in the shared fiber section of the PON. Considering that a wider band would be required, in the future to transmit a large quantity of data over the PON, the second scheme improvable the transmission speed by wavelength multiplexing attracts attention. However, because each ONU has to use a specific wavelength different from that for other ONUs, the diversity of the ONUs increases in the first scheme, as the number of wavelengths to be used in the PON system increases. Consequently, this scheme increases the cost of the ONUs much greater than in the first scheme. In other words, the second scheme requires preparation of more diversified ONUs because all ONUs connected to the OLT transmit and receive optical signals at different wavelengths, respectively. In order to accommodate a new subscriber terminal to the OLT, it is necessary for the second scheme to prepare an ONU that has upstream/downstream wavelengths other than the wavelengths for the existing ONUs and to connect the ONU to an appropriate port of the splitter/filter and, thus, is not easy to add a new ONU.
As a prior art relating to the above ONU diversification problem in the WDM-PON system, for example, Japanese Unexamined Patent Publication No. 2004-222255 (patent document 1) proposes the application of spectrum slicing to share an upstream signal wavelength. In patent document 1, a wide-band light with a wide spectrum is used as an upstream signal to be transmitted from each ONU. Wide-band upstream optical signals are converted into narrow-band optical signals corresponding to ONUs by spectrally slicing the wide-band optical signals that are incoming from a plurality of branch optical fibers by a wavelength splitter, thereby to transmit the narrow-band optical signals to the OLT.
FIG. 9 shows a WDM-PON system setup proposed in patent document 1.
OLT 10 is connected via a wavelength splitter 20 to a plurality of ONUs 30 (30-1 to 30-n). The OLT multiplexes downstream optical signals having individual wavelengths (λd1, λd2, . . . , λdn) allocated to the plurality of ONUs 30 so that an wavelength multiplexed optical signal (λd1+λd2+ . . . +λdn) is transmitted to an optical fiber transmission path 50.
The wavelength multiplexed optical signal is separated into the individual wavelengths by the wavelength splitter 20 and branched away to the branch optical fibers as downstream optical signals with different wavelengths λd1, λd2, . . . , λdn for each ONU. On the other hand, the ONUs 30-1 to 30-n transmit upstream optical signals at the same wavelength λu for all the ONUs.
The wavelength splitter 20 spectrally slices the optical signals incoming from the branch optical fibers into upstream optical signals having different wavelengths λu1, λu2, . . . , λun for each ONU, whereby the incoming signals are converted into a wavelength multiplexed optical signal (λu1+λu2+ . . . +λun), and transmitted to the optical fiber transmission path 50.
FIG. 10 illustrates a relationship between the upstream signal wavelengths and the downstream signal wavelengths used in the WDM-PON system of the above patent document 1.
Out of a wavelength grid in which wavelengths are sequenced at an equal center-to-center distance of Δλd, an arbitrary wavelength λu is selected as the upstream signal wavelength and the remaining wavelengths λd1, λd2 through λdn are used as the downstream wavelengths. The downstream signal wavelengths have a wavelength error within Δλd/2. The wavelength λu is spectrally sliced into multiple wavelengths λu1 to λun corresponding to the ONUs 30-1 to 30-n, respectively. The spectrally sliced upstream signals are sequenced at an equal center-to-center distance of Δλu and have a wavelength error within ±Δλu/2.
In order to accommodate all channels of the upstream optical signals within a bandwidth corresponding to one channel of downstream optical signal, according to patent document 1, the center-to-center distance of Δλd for the wavelengths of the downstream optical signals is defined to be two times or more as much as the center-to-center distance of Δλu for the wavelengths of the upstream optical signals and the center-to-center distance for the wavelengths λu1 to λun of the upstream optical signals is narrowed sufficiently. In patent document 1, it is suggested that an optical filter for wavelength selection is removably installed in the optical transceiver unit of each ONU in order to receive a downstream signal at different wavelength for each ONU.
As a prior art that aimed at effective use of wavelength in the PON system, for example, Japanese Unexamined Patent Publication No. Hei 10-247896 (patent document 2) proposes a PON system in which each ONU dynamically switches a wavelength and a time slot for receiving a downstream signal in response to an instruction from the OLT.
FIG. 11 shows a WDM-PON system setup proposed in patent document 2.
OLT 10 is connected via a wavelength splitter 21 to a plurality of ONUs 30 (30-1 to 30-n). Each ONU 30 transmits upstream signal in a time slot designated from the OLT, using an optical signal at the same wavelength λx. The upstream signals transmitted from each ONU 30 are time division multiplexed (TDM) on an optical fiber 50 and arrive at the OLT. On the other hand, downstream signals are transmitted from the OLT 10 to the optical fiber 50 in a form of a wavelength multiplexed (WDM) signal with wavelengths λ1 and λ2, and branched to each ONU 30 through the optical splitter 21.
FIG. 12 illustrates a method of allocating to each ONU a receiving wavelength and a time slot for receiving downstream signals, proposed in patent document 2.
The OLT 10 transmits a frame A (TF1, λ1) and a frame C (TF2, λ1), each comprising an over header OH and four time slots, using the wavelength λ1, as shown in S1. In parallel with this, the OLT 10 transmits a frame B (TF1, λ2) and a frame D (TF2, λ2), each comprising an over header OH and four time slots T1 to T4, using the wavelength of λ2, as shown in S2. In the OH of each frame, as bandwidth allocation information, information designating ONUs to which the time slots T1 to T4 in the frame of the wavelength λ1 and the time slots T1 to T4 in the frame of the wavelength λ2 are allocated is set.
In the example shown here, the bandwidth allocation information “(λ2) ONU2355” for the frame A indicates that the time slots T1, T2, T3, T4 in the frame B which is transmitted at the wavelength λ2 in parallel with the frame A are allocated to ONU-2, ONU-3, ONU-5, ONU-5, respectively. The information “(λ1)ONU1144” indicates that the time slots T1, T2, T3, T4 in the frame A which is transmitted at the wavelength λ1 are allocated to ONU-1, ONU-1, ONU-4, ONU-4, respectively. In the OH of the frame B, the bandwidth allocation information corresponding to that for the frame A is set. For the subsequent frames C and D as well, ONUs to which the time slots T1, T2, T3, T4 are allocated are specified for each wavelength.
Each of the ONUs is provided with tunable optical filter and receives downstream signals from the OLT at the wavelength λ1 or λ2, selectively. Whether the current receiving wavelength is λ1 or λ2, each ONU can know its allocated time slot during receiving the OH. Accordingly, each ONU can receive downstream signals in the allocated time slot by appropriately switching to the receiving wavelength as instructed from the OLT. In the example shown, the ONU-4 should perform receiving operation in the third and fourth time slots of the frame A (TF1, λ1), in the first and second time slots of the frame D (TF2, λ2), and in the third and fourth time slots of the frame C (TF2, λ1).
As stated above, according to the PON system of patent document 1, selection of an ONU having an unused wavelength or a filter replacement work is required at the time of ONU installation, since an individual wavelength must be fixedly allocated to each ONU for receiving downstream optical signals. Further, this type of PON system involves increased management cost because it requires to record wavelengths currently used in preparation for increasing ONUs later.
On the other hand, the PON system of patent document 2 adopts time division multiplexing at the same wavelength for upstream signals. In principle, it also adopts time division multiplexing for downstream signals, and the number of time slots that can be allocated to each ONU is increased simply by increasing the number of wavelengths applied for downstream signals. Therefore, the PON system of patent document 2 is a modification example to the above-mentioned first scheme, but not a PON system of the second scheme.